1. Field of the Invention
The present invention relates to an oil injected screw compressor adapted to reduce a thrust force acting on a screw rotor.
2. Description of the Related Art
A screw compressor adapted to reduce a thrust force acting on a screw rotor shown in FIGS. 6 to 10 has been heretofore known.
The oil injected screw compressor shown in FIGS. 6 and 7 comprises a compressor body 3 connected to a suction flowpassage 1 at a first end and a discharge flowpassage 2 at a second end oil supply flowpassage 7 connects an oil reservoir 5 which is provided below an oil separating and recovering unit 4 provided in the discharge flowpassage 2, lubricating points such as a bearing, a shaft seal part or the like within the compressor body 3 via an oil pump 6. More specifically, within the compressor body 3, a pair of external and internal screw rotors 11, 12 meshed with each other are rotatably supported by radial bearings 13, 14 on a rotor shaft extending through the compresser body 3, as shown in FIG. 7. In FIG. 7, the left side is the suction side, and the right side is the discharge side. The arrows on the left side indicate an inlet of suction gas, and an arrow on the right side indicates an outlet of discharge gas.
In case of the compressor shown in FIG. 7, a rotor shaft extending to the left side of the external rotor 12 comprises an input shaft 15 which receives a rotational driving force by a motor (not shown). A thrust bearing 16 is provided on the rotor shaft, on the right side of the radial bearing 14 provided on the discharge side of the external rotor 12, and a balance piston 17, for reducing a thrust force acting on the screw rotors 11, 12, i.e, a thrust force acting in a direction from the discharge side toward the suction side is provided on the rotor shaft between the radial bearing 14 and the thrust bearing 16,.
As shown in FIG. 6, the suction flowpassage 1 is at suction pressure P.sub.s, the discharge flowpassage is at discharge pressure P.sub.d, the primary side of the oil pump 6 of the oil supply flowpassage 7 is at discharge pressure P.sub.d, and therefore the secondary side of the oil pump 6 is at oil pressure P.sub.d +.alpha.(.alpha.&gt;0). The relationship between these pressures is P.sub.s &lt;Pd&lt;Pd+.alpha..
Oil having oil pressure P.sub.d +.alpha. leaving oil pump 6 is fed to the bearing and shaft seal (not shown) within the compressor body 3 and acts on the surface on the radial bearing 14 side of the balance piston 17 to reduce the thrust force.
The oil injected screw compressor shown in FIG. 8 basically comprises the same construction as the compressor body 3 shown in FIGS. 6 and 7 which is coupled in tandem by a coupling 21 except that the oil injected screw compressor shown in FIGS. 6 and 7 is of the single stage whereas that shown in FIG. 8 is of the two-stage. Accordingly, parts of the oil injected screw compressor shown in FIG. 8 corresponding to those of the oil injected screw compressor shown in FIGS. 6 and 7 are indicated by the same reference numerals. Particularly, the description is omitted for parts with a subscript a attached thereto, since they correspond to the same-numbered parts in FIGS. 6 and 7.
Compressed gas discharged from the first-stage compressor body 3 flows from a portion marked by to a portion marked by *, is compressed by the second-stage compressor 3a and is discharged to the discharge flowpassage 2. Also in this compressor, the oil pressure P.sub.d +.alpha. a acts on the surface on the radial bearings 14, 14a of the balance pistons 17, 17a.
A screw compressor shown in FIG. 9 is substantially the same as the compressor shown in FIG. 7 except that, the input shaft 15 is arranged on the discharge side, and the balance piston 17 is arranged on the suction side opposite to the input shaft 15. Parts corresponding to each other are indicated by the same reference numbers, the description of which is omitted.
In FIG. 9, pressure is allowed to act on the left surface of the balance piston 17, that is, the surface opposite to the radial bearing 13 to reduce the thrust force.
The oil injected screw compressor shown in FIG. 10 basically comprises the same construction as the compressor body 3 shown in FIG. 9 except that the oil injected screw compressor shown in FIG. 9 is of the single stage whereas that shown in FIG. 10 is of the two-stage. Accordingly, parts of the oil injected screw compressor shown in FIG. 10 which corresponds those of the oil injected screw compressor shown in FIG. 9, are indicated by the same reference numerals. Particularly, a subscript a is attached to the parts in FIG. 10 which correspond to the same parts in FIG. 9, a description of which is omitted.
Similarly to the above, compressed gas discharged from the first-stage compressor body 3 flows from a portion marked by to a portion marked by *, is compressed by the second-stage compressor 3a and is discharged to the discharge flowpassage 2. Also in this compressor, the oil pressure acts on the surface opposite to the radial bearings 13, 13a of the balance pistons 17, 17a.
In this case, a partitioning wall 31 which cuts off pressure is provided between the balance piston 17 and the coupling 21.
In the case of the screw compressor shown in FIGS. 6, 7 described above, the balance piston 17 is arranged adjacent to the radial bearing 14, and the surface on the side of the radial bearing 14 of the balance piston 17 comprises a pressure receiving surface. It has been found that, it is difficult to secure a sufficient surface area for receiving pressure on the balance piston 17. It has also been found that although, the oil pressure P.sub.d +.alpha. always acts on the radial bearings 13, 14 during operation immediately after the start of the compressor or during no-load operation, the load of the compressor is small and therefore the thrust force is small. In such a case, a force greater than that acting on the screw rotors 11, 12, acts on the balance piston 17 to assume a so-called reverse thrust load state so as to press the screw rotors 11, 12 toward the discharge side. A clearance between the end of the screw rotors 11, 12 on the discharge side and a rotor chamber housing them is made as narrow as possible in order to enhance the performance of the compressor. There is a problem in that as the bearing wears, the screw rotors 11, 12 may come in contact with the wall of the rotor chamber, resulting in failure.
In the case of the compressor shown in FIG. 8 in which the same construction as that of the compressor body 3 shown in FIGS. 6, 7 is coupled in tandem, a discharge port of the first-stage compressor body 3 can be communicated with a suction port of the second-stage compressor body 3a by a flowpassage formed within the casing without depending on an external pipe. However, the aforementioned problem may occur also in this case.
In the case of the compressor shown in FIG. 9, since a diameter of the thrust bearing portion on the discharge side is determined depending on a diameter of the input shaft 15 and a diameter of the radial bearing 14, the load capacity of the thrust bearing 16 having a large inside diameter should be employed. As a result, there is a problem in that the load capacity of the thrust bearing 16 cannot be increased.
Further, in the case of the compressor shown in FIG. 10 in which the same construction as that of the compressor body 3 shown in FIG. 9 is coupled in tandem, it is impossible to form a flowpassage within the casing from a discharge port of the first-stage compressor body 3 to a suction port of the second-stage compressor body 3a and therefore an external pipe should be provided. This complicates the construction of the compressor and makes the apparatus bulky. In addition, vibrations and noises caused by a pulsation of the discharge gas from the first-stage compressor body 3 increase.
The present invention solves the problems as noted above with respect to prior art. An object of the present invention is to provide a screw compressor which increases a pressure receiving area of a balance piston, employs a thrust bearing having a large load capacity, removes an occurrence of a reverse thrust load state, has a simple and compact construction, and inhibits vibration and noise.